1. Field of the Invention
The present invention relates to a code plate of an optical rotary encoder.
2. Description of the Related Art
In a known conventional technique, control operations, such as controlling operations of the suspension damping force, controlling operations of the automatic transmission shift position, and steering control operations of the rear wheel of a four-wheel vehicle, are performed by detecting the steering angle, steering speed, and steering direction of a steering wheel mounted to a shaft as a result of rotation of a code plate by means of the steering shaft. In this technique, an optical rotary encoder comprising a combination of a photointerrupter and the code plate disposed around the steering shaft is used.
As shown in FIG. 11, a commonly used code plate has fine rectangular or elliptical light detection patterns (or A, B phase detection patterns) 2 and arcuate light detection patterns (or Z phase detection patterns) 3. The A, B phase detection patterns 2 are used to detect the angle of rotation, direction of rotation, and rotational speed of a rotary substrate 1, which is a non-transparent disk formed from a plastic plate, metallic plate, or the like. The light detection patterns 3 are used to detect the origin and the number of rotations of the rotary substrate 1. However, in such a conventional code plate, the axial line in the longitudinal direction of the A, B phase detection patterns and the sides of the leading edge and the trailing edge of the Z phase detection patterns 3 are both oriented radially from the rotational center 0 of the rotary substrate 1. In this document, light detection pattern is a general term used to cover light-transmitting apertures in a light transmission type rotary encoder and light reflection portions in a light reflection type rotary encoder.
As shown in FIG. 11 illustrating a portion of the code plate, a predetermined number of light-transmitting holes 2, forming the light detection patterns, are formed with the same width and at a constant pitch in the outer periphery of the non-transparent disk 1 formed from, for example, a plastic sheet or a metallic plate. Ordinarily, as shown in FIG. 15, light-transmitting holes 2, forming the light detection patterns, are formed in the non-transparent disk 1 by interposing the non-transparent disk 1 between a punch 44 and a dice 46. The punch 44 is provided with a predetermined number of light-transmitting hole opening pins 43 which can move into and out of the dice 46. The dice 64 has pin receiving holes 45 formed so as to oppose the light-transmitting hole opening pins 43. Then, the light-transmitting hole opening pins 43 move into and beyond the dice 46 to form the light-transmitting holes 2 forming the light detection patterns.
Optical rotary encoders used, for example, to detect the steering angle of a vehicle steering wheel are required to perform angle detection with high precision and resolution in order to perform control operations with high responsivity. In addition, in such optical rotary encoders, a first photointerrupter 4 for A, B phase detection and a second photointerrupter 5 for Z phase detection must often be disposed on the left and right in nonalignment relationship due to the limited space available, as shown in FIG. 12. Although in FIG. 12, the second photointerrupter 5 for A, B phase detection is set on a vertical line X--X which is perpendicular to the surface of a printed board (not shown) having the photointerrupter 5 mounted thereon and which passes through the rotational center O of the rotary substrate 1, while the first photointerrupter 4 for Z phase detection is disposed along a side of the photointerrupter 5, the photointerrupter 4 for Z phase detection can be set on the vertical line X--X passing through the rotational center O of the rotary substrate 1, while the second photointerrupter 5 for A, B phase detection can be placed along a side of the photointerrupter 4.
In the photointerrupters 4 and 5, the light-emitting device and the light-receiving device are combined in a predetermined arrangement such that light from the light-emitting device impinges upon the light-receiving device by passing through the code plate. The light-transmission type rotary encoder used has the light-emitting device 6 disposed in opposing relationship with the light-receiving device 7 on the same axis. As is clear from FIG. 13, in the case of a light transmission type rotary encoder, the light-emitting device 6 and the light-receiving device 7 are disposed on the front and back sides of the rotary substrate 1, with an optical axis Z--Z being oriented perpendicular to a line Y--Y defining the surface of the rotary substrate 1. Although not shown, in the case of a reflection type rotary encoder, the light-emitting device and the light-receiving device are set at locations where total refection occurs through the reflecting surfaces of the rotary substrate 1. The light-receiving device 7 has at least two light-receiving surfaces 7a and 7b formed in the horizontal direction, so as to be disposed toward the left and right in FIG. 12. Pulse signals (A phase signal and B phase signal) which are 90 degrees out of phase can be detected from the first light-receiving surface 7a and the second light-receiving surface 7b, respectively. In such an encoder, a light-receiving device with four light-receiving surfaces 7a, 7b, 7a', and 7b' is used to allow high-precision signal detection by the so-called differential output. The sides of each of the light-receiving surfaces 7a and 7b are formed parallel to the vertical line X--X passing through the rotational center O of the rotary substrate 1. Terminals 20 of the photointerrupter 5 are set parallel to the vertical line X--X.
Taking as an example the light-transmission type rotary encoder in which the second photointerrupter 5 for Z phase detection is set on the vertical line X--X passing through the rotational center O of the rotary substrate 1 and the first photointerrupter for A, B phase detection is disposed along a side of the photointerrupter 5, a spot S is formed by light emitted from the light-emitting device 6, passing through the A, B phase detection patterns 2, and impinging upon the set surfaces of the light-receiving device 7. The shape of the spot S changes in accordance with the angle of rotation of the rotary substrate 1, as indicated by cross-hatching in FIGS. 14A to 14D. FIG. 14A shows the shape formed when the A, B phase detection patterns 2 with a two-degree pitch exactly matches the set positions of the light-emitting device 6 and the light-receiving device 7 (defined as the initial position). FIG. 14B shows the shapes formed when the rotary substrate 1 has been rotated 0.5 degrees counterclockwise from the initial position. FIG. 14C shows the shapes formed when the rotary substrate 1 has been rotated 1.0 degree counterclockwise from the initial position. FIG. 14D shows the shapes formed when the rotary substrate 1 has been rotated 1.5 degrees counterclockwise from the initial position.
As is clear from the FIGS. 14A to 14C, in the optical rotary encoder of the present embodiment, the two light-receiving surfaces 7a and 7b are arranged in the horizontal direction, while the shape of the spot S formed by the light impinging upon the light-receiving surfaces is tilted in correspondence with tilting angle .theta. of the A, B phase detection patterns 2, causing the light spot 2 to cross the light-receiving surfaces 7a and 7b obliquely as the code plate rotates. Therefore, when the encoder is constructed to output the A phase and B phase signals with a phase difference of 90 degrees as the untilted light spot on the two light-receiving surfaces 7a and 7b moves, the phase difference of the A phase signal and the B phase signal is not 90 degrees, as illustrated in FIG. 14E, and, in addition, the duty ratio between the A phase and B phase is not 50%. For this reason, limitations are placed on how the A, B phase detection first photointerrupter 4 can be arranged, thus making it difficult to design the optical rotor encoder and tending to create errors in the rotational angle detection, which may seriously affect the accuracy of the various control operations. This problem becomes more noticeable, the smaller the pitch of the plurality of light detection patterns is made to increase resolution. The same problem occurs when detecting a signal by the so-called differential output where the A, B phase detection device used is one having four light-receiving surfaces 7a, 7b, 7a', and 7b', so that the light-receiving surface 7a' is connected to the light-receiving surface 7a so as to generate opposite phases and the light-receiving surface 7b' is connected to the light-receiving surface 7b so as to generate opposite phases.
Such a problem can be overcome by changing the forms and arrangement of the light-receiving surfaces 7a and 7b and using the light-receiving device 7 which outputs an A phase signal and a B phase signal which are out of phase by 90 degrees with respect to the tilted light spot S. However, this is far from being a practical solution, since developing and producing a special-purpose light-receiving device 7 in accordance with the type of encoder results in expensive rotary encoders.
In the second photointerrupter 5 for Z phase detection set on the vertical line X--X passing through the rotational center O of the rotary substrate 1, the above-described problem does not arise even when the sides of the leading edge and the trailing edge of the Z phase detection patterns 3 are formed radially from the rotational center O of the rotary substrate 1, since the light spot crosses the light-receiving surfaces of the light-receiving device, while being kept perpendicular thereto.